Apparatus for manufacturing semiconductor materials



May'3, 1960 B. CORNELISON 2,935,335

APPARATUS FOR MANUFACTURING SEMICONDUCTOR MATERIALS Fi led March 25,1955 2 Sheets-Sheet 1 I N V ENTOR 50m Coma/saw ATTORNEYS APPARATUS FORMANUFACTURING SEMICONDUCTOR MATERIALS Filed larch 25, 1955 May 3, 1960B. CORNELISON 2 Sheets-Sheet 2 INVENTOR F l G. 3

BY mww yw ATTORNEYS 2,935,335 APPARATUS FoR MANUFACTURING SEMICONDUCTORMATERIALS Boyd Cornelison, Dallas, Tex., assignor to TexaslnstrumeritsIncorporated, Dallas, Te-x., a corporation of Delaware Application March25, 1955, Serial No. 496,895

2 Claims. -(Cl. 23-273) This invention relates to improvements inapparatus for the production of the semiconductormaterials usedinmanufacturing semiconductor devices such as rectiafiers andtransistors. More particularly, this invention "relates to improvementsin the machines used toproduce Tsemiconductor segment which in turn,depends on the crystal lattice structure of the monocrystalline massfrom .which it has been out. In order to v produce large single crystalsof the desired specific structure wherein the lattice defects andimpurity concentrations are controlled as to location and. area,intricate machines have-becn developed. Most of these machines producesemiconductor crystals by the method known asthe pulling method.

In the pulling method of growing semicpnductor crystals, a quantity ofextremely pure semiconductor material, such as germanium or silicon, ismelted in an atmosphere ofinert gas or in a vacuum. Next, a smallpieceof a like type of semiconductor material with a single crystallattice structure, called a seed, is brought into rotating contactwiththe molten material. The rotating seedis then withdrawn fromthe meltat arate substantially equal to the rate of solidification of the moltenmaterial adhering to the seed. While the seed is .being withdrawn, asingle crystal ofsemicond ctor material of the same lattice structure asthe seed forms as the material of the melt solidifiesor grows at thebottom of the seed. Through careful and precise control of temperature,withdrawal rate, and the-introduction into the melt of exact quantitiesof impuritiesat specific times, the size, shape and electricalcharacteristics of the. growing crystalcan be controlled. To illustratethe precise control required, the machines used inthis pulli ng ,methodof manufacturing semiconductor crystals must jbecapable of controllingthe temperature. of thesemicon- ,ductorsmaterial to within 10.2 C. ofany desired temperature over a range of from less than 900 C. to morethan 1800 C. Also, these machines must be capable .-of controlling thewithdrawal rate of theseed to within :':0.005 mil/second at a high rateof pull such as 5.0 mils/ ,second and to within 1 0.001 mil/second at alow rate oi pull such as 0.005 mil/second.

,Crystal pulling machines in use at the present time vusually employ aradio-frequency induction furnacewith a thermocouple controlledservo-system to regulate the --radio-frequency generator power outputand thus the ,temperature of the furnace.

.In ordertopontrol-the withdrawal rate, the pulling -rncchanisms ofpresent crystal pulling machines usually v; m s ist of a, D.-C. motordriving ,a lead screwthrough Tia reduction. gear ,ttrain arrangement.:Asthe lead .screw is turned, a guided platform threaded to the leadscrew is caused to move up or down. Rotatably attachedto this platformis a rod with the seed crystal aflixed to its lower end portion. Thewithdrawal rate of the seed crystal from the melt is controlled througha servomechanism which operates to govern the speed of D.-C. motor bymeans of a thyra'tron-rectifiermotorspeed controlof the type describedby H. H. Leigh in Simplified Thyratron Motor Control, General ElectricReview, September 1946 Although the accuracy ofthis type of pull-ratecontrol has generally been found to be satisfactory for use in themanufacture of production quantities of single crystals of semiconductormaterials, it has certain disadvantages. For example, the thyratronmotor speed control device with the accuracy of control which isrequired for this particular application uses from five to eightelectron tubes with their associated components, each a potential pointof failure and, in addition, making such a control is some, whatexpensive. Another undesirable feature is that voltage regulatorequipment must be used in series with the power lines if the speed ofthe DC. motor is to be unaffected by voltage fluctuations in the primaryelectrical power. Still another undesirable feature is size sinceathyratron control unit suitable for use in. semiconductor crystalpulling machines occupies a volume ofapproximately nine and one-halfcubic feet. A still furtherdisadvantage is that D.-C.,motors will notrun below a certain minimum speed and no control can be provided betweenthis speed and zero output nor can the lower speeds at which the motorwill run be accurately controlled.

Also, there is a limited range of speed variation in such controls, themaximum ratio being approximately 50:1

Accordingly, it is an object of the present invention to provide apullrate control system for semiconductor crystal pulling machines whichis less susceptible to operational failures than the type of controlspresently used.

It is another object of the present invention to provide a pull-ratecontrol for semiconductor crystal pulling machines which is much lessexpensive thancontrolspres- ,ently used.

It is another object of the present invention toprovidea pull-ratecontrol mechanism for semiconductor crystal pulling machines which willbe unaflfected by voltage fluctuations in the primary electrical powerlines. It is a further object of the present-invention to provide apull-rate control mechanism for semiconductor crystal pulling machineswhich may bevaried continuously and accurately from a zero speed to itsmaximumspecd to provide pulling rates from zero to approximately ,15mils/second. V

It is still another object of the present inventionto provide apull-rate control system for semiconductor crystal pulling machineswhich is much-smaller than. control systems now in use.

.The above objectstogetherwith further objects and details of thepull-rate control system of the present invention will become apparentfrom the following description when taken in conjunction, with theaccompanying drawings in which: Figure 1 is a diagrammaticrepresentation of the pull- ,rate mechanism and control system of thepresent invention;

Figure 2 is a diagrammatic illustration of the linkage of avariablespeed reduction unit suitable for use. in the pullrate control system ofthe present invention;

Figure 3 is a further diagrammatic representationrof the linkage .ofFigure 2 illustrating the manner in which linkageis adjusted to vary thesystem pull-rate; and

,Figuret4 is a schematic diagram of a control servos stem su tabl f sait th Pr sen in ention.

a into the sealed container 4 of the induction furnace where thecrystalis to be grown. The pull rod 3 is rotatably suspended by the thrustbearing 5 from the movable platform 6. Also mounted on the platform 6 isa small variable speed motor 7 which, through the worm gear 8, drivesthe gear 8a keyed to the pull rod 3 to impart a spinning motion to theseed crystal 1 and thus provide the necessary stirring action in themolten semiconductor material in the furnace. Vertical motion of theplatform 6 is supplied by rotation of the lead screw 9 which is inthreaded engagement with the platform 6 at 10. The platform 6 is guidedin a true vertical plane of motion by the two vertical guide rods 11 and12 slidably journaled in the platform sleeve 13 and another such sleevenot shown. A close fit of the guide rods 11 and 12 in the long sleevesof the platform prevents wobble of the platform and the consequentbinding of the lead screw 9 which could conceivably producemalformations in the growing crystal due to uneven or jerky pullingmotion. The lead screw 9 is turned by the output shaft 14 of theconstant-ratio reduction gear box 15 through the bevel gears 16 and thereduction gear train comprised of'the gears 17, 18, 19, and driving gear20 keyed to the lead screw 9. The input shaft (not shown) of theconstantratio reduction gear box 15 is driven from the variableratiospeed reduction unit 21 by means of the output shaft 22 and the gears 23and 24 aflixed to the shafts of the two units 21 and 15 respectively.The primary drive of the entire pull system is supplied by thesynchronous motor 25 through a chain or timing belt 26 connecting thepulley 27 of the motor 25 to the pulley 28 fixed to the input shaft (notshown) of the variable-ratio speed reduction unit 21. Although varioustypes of motors other than synchronous motors may be used, this typemotor is most desirable because its speed is affected only by linefrequency changes and not by line voltage changes.

It is by means of the variable-ratio speed reduction unit 21 that thepull rate is governed. A servo-mechanism is used to control the rotationof the rotor shaft 64 of the balance motor 29 which, through the wormgear coupling 30 to the control shaft 31 of the variable speed reductionunit 21, controls the speed of the output shaft 22 of the variable speedreduction unit 21. A variable speed reduction unit of the type suitablefor use in the present invention is manufactured by Revco Incorporatedof Minneapolis, Minnesota, under the trade-name Zero- Max. Figures 2 and3 illustrate the mechanism of such a device.

In Figure 2, the input shaft of the variable speed reduction unit 21 isdesignated by the numeral 41. Attached to the input shaft 41 is a crankdisk 32 with a bar or link 33 pivoted on the crank pin 79 near its outeredge. The other end of the link 33 is connected to two other links 34and 35 by the pin 36 in such a way that all three links are free topivot about the pin 36. The control link 34 is pivoted at its other endon the pin 37 of the adjustable pivot block 38. Movement of the pivotpin 37 and block 38 is restrained by the threaded engagement of theblock 38 to the control shaft 31. The drive link 35 is connected to theouter plate 66 of some overrunning clutch mechanism such as an internalcone friction clutch 39, which is well known in the art, while the innerplate 65 of clutch 39 is attached to the output shaft 22 of the speedreduction unit 21.

This linkage constitutes, in essence, two four-bar linkages with acommon link 34 adjustably connected to the control shaft 31 of thevariable speed mechanism. As illustrated by the dashed lines of Figure2, when the input shaft 41 and crank disk 32 rotate, motion istransmitted through the link 33 to the junction point 36 of the threelinks. Because the pivot point 37 is restrained, the common point ofconnection 36 of the three links-i restricted to an oscillating motionalong the are 40 centered at the pivot point 37. This motion causes thedrive link 35 to oscillate which, in turn, oscillates the outer plate 66of the overrunning clutch 39 through part of a revolution as indicatedat 67. The clutch 39 engages and turns the inner clutch plate 65together with the output shaft 22 keyed thereto in one direction ofrotation but disengages on the return motion and, consequently, does notturn inner clutch plate 65 and output shaft 22 in the other. directionof rotation. Thus, each revolution of the input shaft 41 turns theoutput shaft 22 through part of a revolution. By placing a number ofsuch mechanisms on the input and output shafts and arranging them sothat the links are at equally spaced points about the input shaft, arelatively smooth, continuous rotation is transmitted to the outputshaft.

Figure 3 illustrates the method of changing the speed reduction ratio ofthe unit 21 by changing the position of the pivot point 37 throughrotation of the control shaft 31. As the control shaft 31 is rotated,the pivot block 38 is caused to move up or down by its threadedengagement with the control shaft 31. In Figure 3, the pivot point 37has been moved up from the position illustrated in Figure 2 so that themovement imparted to the overriding clutch by the oscillating motion oflink 35 is of a greater amplitude as indicated at 68. Thus, therotational speed of the output shaft 22 has been increased although therotational speed of the input shaft 41 remains the same. The describedlinkage provides a very satisfactory means for varying the speed ofoutput shaft 22 but this invention is not to be construed as limited tothe use of the four-bar linkage arrangement since other mechanisms suchas a hydraulic coupling can be used for this purpose.

As stated above, the pull rate of the system of the present invention isselected and varied by a servo'mechanism operating the motor 29 to turnthe control shaft 31 of the variable-ratio speed reduction unit 21. Aservosystem well suited for use with the present invention isillustrated in Figure 4.

In the servo-system illustrated in Figure 4, an A.-C. voltage from powerlines 44 and 45 through the transformer 46 is impressed across the endterminals of two precision potentiometers 42 and 43. Because these twopotentiometers are connected in parallel across the secondary coil 47 ofthe transformer 46, their voltages are of exactly the same phase.Therefore, if the sliders 48 and 49 of the potentiometers 42 and 43contact their respective potentiometers at the same relative point, novoltage will be supplied to the input terminals 50 and 51 of theamplifier 52 since the sliders to which these terminals are connectedwill be at the same potential. However, if one of these sliders 48 isdisplaced, a voltage is developed between the terminals 50 and 51. Thephase of this voltage will be determined by the direction ofdisplacement of the slider 48 and its amplitude will be determined bythe amount of displacement. For example, if

the slider 48 is displaced toward the top of the drawing,

the signal across the terminals 50 and 51 will be in phase with thesignal across the secondary coil 47 of the transformer 46. However, ifslider 48 is displaced toward the bottom of the drawing, the phase ofthe voltage to the terminals 50 and 51 will be reversed and it will beout of phase with the voltage of coil 47.

Any signal voltage at terminals 50 and 51 is amplified by the amplifier52 and fed to the grids of two triode tubes 53 and 54. The tubes 53' and54 have their cathodes connected together and to ground through thecommon cathode resistor 55. The grids likewise are connected togetherand to ground through a common grid resistor 56. The plates of the twotubes are connected to opposite ends of another secondary coil 57 of thesame transformer 46 which supplies voltage to the two precisionpotentiometers 42 and'43. The center tap 59 of the secondary coil 57 isconnected to ground through the rotor coil 58 of thejbalance motor 29 sothat theplate voltages of the twoxtubes 53' and 54 are 180" out ofphase. Since the grid signal voltage is derived from the sametransformer 46 as the plate voltage, the grid voltage on the tubes 53and 54 will always be in phase with the plate voltage of one'of'thetubes and 180 out of phase with the other and only the tube in which.the grid voltage and plate voltage are in phase will conduct currents.The direction of displacement of the slider 48 determines the phase ofthe grid voltage to the tubes 53 and 54 and, therefore, the direction ofdisplacement of the slider 48 determines which of the tubes, 53 or 54,conducts.

By referring to the diagram of Figure 4, it can easily be seen that anycurrent which flows in the rotor coil 58 of the balance motor 29 mustalso flow through either the upper or lower half of the transformersecondary coil 57. Thus, the current of the rotor coil 58 must be of oneof two phases, each 180 different from the other, and the phase of thiscurrent depends on the part of coil 57 in which current is flowing orwhich tube, 53 or 54, is conducting. In other words, the phase ofthecurrent in the rotor coil 58 of the balance motor 29 is determined bythe direction of displacement of the slider 48. As shown in Figure 4,slider 48 is displaced by means of the cam 62 and follower mechanism 63but any of several means may be used for this purpose.

The voltage of the stator coil 60 of the balance motor 29 is producedfrom the power supply lines 44 and 45 and, were it not for the condenser61 inserted in the line to the stator coil 60, the voltage wouldbe-either in phase or 180 out of phase with the rotor coil 58 voltage.However, the condenser 61 shifts the stator coil 60 voltage in phase by90. Since any currents which flow in the rotor and stator coils 58 and60 of the balance motor 29 will always be 90 out of phase with eachother, the rotor of the balance motor 29 will turn when current flows inthe rotor coil 58 and its direction of rotation will be determined bywhether the current in the rotor coil 58 is lagging or leading thecurrent of the stator coil 60.

Coupled to the rotor shaft 64 of the balance motor 29 are the controlshaft 31 of the variable speed reduction unit 21 and the slider 49 ofthe follow-up potentiometer 43. The coupling of slider 49 to shaft 64may be either direct or by means such as gearing depending upon thenumber of turns on shaft 31 and the construction of potentiometer 43.When current flows in the rotor coil 58 as a result of displacement ofthe slider 48, the rotor shaft 64 turns and there is produced a changein the pullrate of the system through rotation of the control shaft 31of the variable speed reduction unit 21. The rotor shaft 64 willcontinue to turn and the pull-rate of the system will continue to changeuntil the slider 49 coupled to the rotor shaft 64 is displaced from itsoriginal contact point on the potentiometer 43 to a contact point atwhich its potential is exactly the same as the potential of the slider48. At this point, nofurther signal is developed between terminals 50and 51 and thus no grid voltage signal is fed to the tubes 53 and 54.Without a grid voltage, no current will pass through either of the tubesand, consequently, no current will flow in the rotor coil 58. Of course,when current no longer flows in the rotor coil 58, the control shaft 31no longer turns and the pull-rate of the system ceases to change.

By means such as the cam 62 and follower 63 mechanism, wherein thevariations in radius along the circumference of cam 62 are proportionalto the pull-rate changes to be produced, the signals for a predeterminedprogram of pull-rate changes can be introduced into the system. Using asimilar cam and follower to produce signals for temperature changes in atemperature control servosystem of the crystal pulling machine and asmall motor to drive both cams at the same speed, the crystal pullingprocedure can be made into an almost completely automatic process.

areas "s Returning now to Figure 1', provision has been made so that theseed may be raised or lowered manually at a greater speed. The gears 18and 19 are first disengaged from the gears 17 and 20 respectively by adownward motion of the handle 69 of the lever .70 pivoted on-the-pin 71to raise the yolk 72 and collar 73 attached to t-heshaft 74. As theshaft 74 is lifted, thegea'r-s' 18 and 19 also connected thereto arelifted out ofmesh with the gears 17 and 20. The crank 75 is then movedin an axial direction against spring 76 to engage the bevel gear 77 withthe bevel gear 78 attached to the lead screw 9. By rotating the crank75, the lead screw 9 is turned and the platform 6 and pull rod 3 may beraised or lowered to the desired position. When the crank 75 isreleased, spring 76, which was compressed by the axial movement of thecrank 75, returns the crank 75 to its original axial position thusdisengaging the bevel gears 77 and 78. The handle 69 may then be raisedengaging the gears 19 and 20 and 17 and 18 to begin, as described above,the con trolled movement of the pull rod and its attached crystal.

Thus, there has been described a pull-rate mechanism for semiconductorcrystal pulling machines which, compared with other such mechanismspresently in use, has the advantages of being smaller and more compact,less expensive, less susceptible to failures, unaifected by voltagefluctuations of the primary power and capable of producing continuouscontrol from a zero pull rate to its maximum pull rate with an accuracyof :0.001 mil/ second at pull rates above zero.

It is recognized that many changes, substitutions, and alterations inthe illustrated embodiment, still within the scope and spirit of thepresent invention, will suggest themselves to those skilled in the art.Therefore, it is to be understood that the scope of the presentinvention is not to be limited to the embodiment illustrated herein butis to be limited only as set forth in the appended claims.

What is claimed is:

1. In an apparaus for producing single crystals of semiconductormaterial, a mechanism for driving the crystal seed support andwithdrawal means at a continuously variable controllable speedcomprising in combination a synchronous motor, a continuously variableratio transmission means driven by said motor and driving the crystalseed support and withdrawal means, and a servomechanism controlling thetransmission ratio of said transmission means, said transmission meanscomprising an input shaft, a plurality of discs keyed to said inputshaft, a plurality of unidirectional clutches each with an inner and anouter plate, said inner plate fixed to an output shaft, a plurality oflinkages each interconnecting one of said discs and one of said outerplates, and a control shaft coupled both to said interconnectinglinkages and to said servomechanism.

2. In an apparatus for producing single crystals of semiconductormaterial, a mechanism for driving the crystal seed support andwithdrawal means at a continuously variable controllable speedcomprising in combination a synchronous motor, a continuously variableratio transmission means driven by said motor and driving the crystalseed support and withdrawal means, and a servomechanism controlling thetransmission ratio of said transmission means, said transmission meanscomprising an input shaft, a plurality of discs keyed to said inputshaft, a plurality of unidirectional clutches each with an inner and anouter plate, said inner plate fixed to an output shaft, a plurality oflinkages each interconnecting one of said discs and one of said outerplates, and a control shaft coupled both to said interconnectinglinkages and to said servomechanism, each of said plurality of linkagesbeing comprised of two 4-bar linkages with a link common to both of said4-bar linkages and each of said common links adjustably coupled to saidcontrol shaft.

(References on following page) 7 8 References Cited in the file of thispatent 2,743,200 Hannay Apr. 24, 1956 UNITED STATES PATENTS 2,768,914Buchler Oct. 30, 19 56 3 Bugle p 1,968,030 De Filippis July 31, 19342,508,639 Field May 23, 1950 5 OTHER REFERENCES 2,683,676 Little et a1.July 13, 1954 Transistor Technology, Bell Laboratory and Western2,692,510 Gille Oct. 26, 1954 Electric 00., Inc., vol. 1, chapt. 5 to 7.t

1. IN AN APPARATUS FOR PRODUCING SINGLE CRYSTALS OF SEMICONDUCTORMATERIAL, A MECHANISM FOR DRIVING THE CRYSTAL SEED SUPPORT ANDWITHDRAWAL MEANS AT A CONTINUOUSLY VARIABLE CONTROLLABLE SPEEDCOMPRISING IN COMBINATION A SYNCHRONOUS MOTOR, A CONTINUOUSLY VARIABLERATIO TRANSMISSION MEANS DRIVENN BY SAID MOTOR AND DRIVING THE CRYSTALSEED SUPPORT AND WITHDRAWAL MEANS, AND A SERVOMECHANISM CONTROLLING THETRANSMISSION RATIO OF SAID TRANSMISSION MEANS, SAID TRANSMISSION MEANSCOMPRISING AN INPUT SHAFT, A PLURALITY OF DISCS KEYED TO SAID INPUTSHAFT, A PLURALITY OF UNDIRECTIONAL CLUTCHES EACH